Sputtering, alternatively called physical vapor deposition (PVD), is commonly used in depositing layers of metals and related materials in the fabrication of semiconductor integrated circuits. Recent technology developed for copper metallization in silicon integrated circuits has emphasized sputtering of refractory metals such as tantalum for use as a barrier layer in a interconnect hole structure etched into a dielectric and sputtering of copper for use as a seed layer for the final electroplating of copper to fill the hole. The requirements placed upon sputtering have intensified as the diameter of the interconnect hole has shrunk to below 100 nm and as the aspect ratio of holes has increased to 5 and above.
Advanced sputter reactors have been developed with complexly shaped targets and inductive power sources, which for the most part are intended to increase the ionization fraction of the sputtered atoms. Thereby, biasing of the wafer attracts the ionized sputtered atoms to deep within holes and also to sputter etch overhangs and undesired layers at the bottom of the holes. However, fairly conventional sputter reactors continue to be used even in advanced applications because of their simplicity and low cost. The conventional sputter reactor is modified with sophisticated magnetics to achieve many of the performance characteristics of the more complex sputter reactors.
Gung et al in U.S. Pat. No. 6,610,184, incorporated herein by reference, hereafter Gung, disclose a plasma sputtering reactor 10 illustrated in the schematic cross-section view of FIG. 1. A vacuum chamber 12 includes generally cylindrical sidewalls 14, which are electrically grounded. Typically, an unillustrated grounded replaceable shield and sometimes an additional floating shield are located inside the sidewalls 14 to protect them from being sputter coated, but they act as chamber sidewalls except for holding a vacuum. A sputtering target 16 having at least a surface layer composed of the metal to be sputtered is sealed to the chamber 12 through an electrical isolator 18. A pedestal electrode 22 supports a wafer 24 to be sputter coated in parallel opposition to the target 16. A processing space is defined between the target 16 and the wafer 24 inside of the shields.
A sputtering working gas, preferably argon, is metered into the chamber from a gas supply 26 through a mass flow controller 28. An unillustrated vacuum pumping system maintains the interior of the chamber 12 at a very low base pressure of typically 10−8 Torr or less. During plasma ignition, the argon pressure is supplied in an amount producing a chamber pressure of approximately 5 milliTorr, but as will be explained later the pressure is thereafter decreased. A DC power supply 34 negatively biases the target 16 to approximately −600 VDC causing the argon working gas to be excited into a plasma containing electrons and positive argon ions. The positive argon ions are attracted to the negatively biased target 16 and sputter metal atoms from the target.
The invention is particularly useful with self-ionized plasma (SIP) sputtering in which a small nested magnetron 36 is supported on an unillustrated back plate behind the target 16. The chamber 12 and target 16 are generally circularly symmetric about a central axis 38. The SIP magnetron 36 includes an inner magnet pole 40 of a first vertical magnetic polarity and a surrounding outer magnet pole 42 of the opposed second vertical magnetic polarity. Both poles are supported by and magnetically coupled through a magnetic yoke 44. The yoke 44 is fixed to a rotation arm 46 supported on a rotation shaft 48 extending along the central axis 38. A motor 50 connected to the shaft 48 causes the magnetron 36 to rotate about the central axis 38.
In an unbalanced magnetron, the outer pole 42 has a total magnetic flux integrated over its area that is larger than that produced by the inner pole 40, preferably having a ratio of the magnetic intensities of at least 150%. The opposed magnetic poles 40, 42 create a magnetic field inside the chamber 12 that is generally semi-toroidal with strong components parallel and close to the face of the target 16 to create a high-density plasma there to thereby increase the sputtering rate and increase the ionization fraction of the sputtered metal atoms. Because the outer pole 42 is magnetically stronger than the inner pole 40, a fraction of the magnetic field from the outer pole 42 projects far towards the pedestal 22 before it loops back to behind the outer pole 42 to complete the magnetic circuit.
An RF power supply 54, for example, having a frequency of 13.56 MHz is connected to the pedestal electrode 22 to create a negative self-bias on the wafer 24. The bias attracts the positively charged metal atoms across the sheath of the adjacent plasma, thereby coating the sides and bottoms of high aspect-ratio holes in the wafer, such as, inter-level vias.
In SIP sputtering, the magnetron is small and has a high magnetic strength and a high amount of DC power is applied to the target so that the plasma density rises to above 1010 cm−3 near the target 16. In the presence of this plasma density, a large number of sputtered atoms are ionized into positively charged metal ions. The metal ion density is high enough that a large number of them are attracted back to the target to sputter yet further metal ions. As a result, the metal ions can at least partially replace the argon ions as the effective working species in the sputtering process. That is, the argon pressure can be reduced. The reduced pressure has the advantage of reducing scattering and deionization of the metal ions. For copper sputtering, under some conditions it is possible in a process called sustained self-sputtering (SSS) to completely eliminate the argon working gas once the plasma has been ignited. For aluminum or tungsten sputtering, SSS is not possible, but the argon pressure can be substantially reduced from the pressures used in conventional sputtering, for example, to less than 1 milliTorr.
An auxiliary array 60 of permanent magnets 62 is positioned around the chamber sidewalls 14 and is generally positioned in the half of the processing space towards the wafer 24. The auxiliary magnets 62 have the same first vertical magnetic polarity as the outer pole 42 of the nested magnetron 36 so as to draw down the unbalanced portion of the magnetic field from the outer pole 42. In the embodiment described in detail below, there are eight permanent magnets, but any number of four or more distributed around the central axis 38 would provide similarly good results. It is possible to place the auxiliary magnets 62 inside the chamber sidewalls 14 but preferably outside the thin sidewall shield to increase their effective strength in the processing region. However, placement outside the sidewalls 14 is preferred for overall processing results.
The auxiliary magnet array 62 is generally symmetrically disposed about the central axis 38 to produce a circularly symmetric magnetic field. On the other hand, the nested magnetron 36 has a magnetic field distribution is asymmetrically disposed about the central axis 38 although, when it is averaged over the rotation time, it becomes symmetric. There are many forms of the nested magnetron 36. The simplest though less preferred form has a button center magnetic pole 40 surround by an circularly annular outer magnetic pole 42 such that its field is symmetric about an axis displaced from the chamber axis 38 and the nested magnetron axis is rotated about the chamber axis 38. One such nested magnetron has a triangular shape with an apex near the central axis 38 and a base near the periphery of the target 16. This shape is particularly advantageous because the time average of the magnetic field is more uniform than for a circular nested magnetron.
Gung describes the effects of their magnetic elements. The unbalanced magnetron 36 creates a semi-toroidal magnetic field BM that is generally parallel to the sputtering face of the target 16 to thereby trap electrons, increase the plasma density, and hence increase the sputtering rate. Because of the imbalance, a substantial unmatched magnetic field emanates from the outer pole 42 creating both a return magnetic field BA1, which projects into the chamber 12 near the chamber center 38 but returns to the back of the magnetron 36, and a sidewall magnetic field BA2 near the chamber sidewall 14. The sidewall magnetic field BA2 is drawn toward the similarly polarized auxiliary array 62 before it returns to the back of the magnetron 42. Gung describes the beneficial effects of such an arrangement as extending the plasma and guiding the ionized sputter particles towards the wafer 24. He further describes the improved radial uniformity of deposition of copper films.
The Gung configuration has been advantageously applied to copper deposition, particularly for a thin copper seed layer into a narrow via hole formed through an inter-level dielectric for forming a vertical interconnect as well as for horizontal interconnects in the commercially important dual-damascene structure. The copper seed layer is used as a seed and electroplating layer for the subsequent filling of the via hole by electrochemical plating (ECP). In this application, overhang is a considerable problem. On the other hand, when the Gung configuration is applied to sputtering a tantalum barrier layer between the walls of the via hole and the copper seed layer, the resulting uniformity was not completely satisfactory. In this barrier application, sidewall coverage and uniformity deep in the via hole is more important.